Suspending Lettuce Type Influences Recoverability and Radiation Sensitivity of Escherichia coli O157:H7

Abstract

An outbreak strain of Escherichia coli O157:H7 was inoculated onto closely related but structurally distinct types of lettuce (Lactuca sativa): Boston (butterhead lettuce), iceberg (crisphead lettuce), and green leaf and red leaf (colored variants of looseleaf lettuce). The E. coli O157:H7 was inoculated either onto the surface of cut leaf pieces or into a homogenized leaf suspension. Samples were gamma irradiated, and the radiation sensitivity of the inoculated bacteria was expressed as a D-value (the amount of ionizing radiation necessary to reduce the bacterial population by 90% [kGy]). The recovery of bacteria from nonirradiated leaf pieces was also measured. When inoculated onto the leaf surface, E. coli O157:H7 had significantly stronger radiation sensitivity on red leaf lettuce (D = 0.119 +/- 0.004 [standard error]) and green leaf lettuce (D = 0.123 +/- 0.003) than on iceberg lettuce (D = 0.136 +/- 0.004) or Boston lettuce (D = 0.140 +/- 0.003). When E. coli O157:H7 was inoculated into a homogenized leaf suspension, its sensitivity was significantly stronger on iceberg lettuce (D = 0.092 +/- 0.002) than on green leaf lettuce (D = 0.326 +/- 0.012), Boston lettuce (D = 0.331 +/- 0.009), or red leaf lettuce (D = 0.339 +/- 0.010), with a threefold difference. Significantly fewer bacteria were recovered from the surface of iceberg lettuce than from the surfaces of the other types of lettuce examined. Following radiation doses of up to 0.5 kGy, the texture (maximum shear strength) of lettuce leaves was measured along the midrib and along the leaf edge for each type of lettuce. There was no meaningful change in texture for any type of lettuce for either leaf section examined at any dose up to 0.5 kGy. These data show (i) that relatively subtle differences between lettuce types can significantly influence the radiation sensitivity of associated pathogenic bacteria and (ii) that doses of up to 0.5 kGy do not soften lettuce leaves.

Simulation of Gamma-Ray Irradiation of Lettuce Leaves in a 137Cs Irradiator Using MCNP

“…[11][12][13] Also, their sensory attributes were not affected by irradiation up to 1 and 2 kGy, respectively. In August 2008, the Food and Drug Administration (FDA) allowed the use of ionizing radiation (up to 3 kGy) to make lettuce and spinach safer and delay spoilage.…”

“…Published reports have shown irradiation to be an efficient method for eliminating pathogens from the internal spaces of lettuce and spinach leaf pieces 15) and the D 10 value in the internal regions is three times higher than the value at the surface, 11) which means pathogens in the internal region could be more resistant than the ones on the surface. Even the radiation sensitivity of pathogens appeared to vary with the variety of fresh produce.…”

Shelf life of irradiated minimally processed (MP) watercress (Nasturtium officinale)

“…NIEMIRA et al 20 showed that doses up to 0.5 kGy did not soften lettuce leaves of four varieties (Boston, Iceberg, Green reduced by 1-4 log 10 . Faecal coliforms and Salmonella spp.…”

“…NIEMIRA et al 20 showed that doses up to 0.5 kGy do not significantly change the texture of four different varieties of lettuce. FOLEY et al 10 and HAGENMAIER and BAKER 13 did not observe any changes in the appearance and texture of iceberg lettuce irradiated at 0.15-0.5 kGy.…”

The Potential of Food Irradiation: Benefits and Limitations

“…However these maximum values depend on the type of products and might modify with new, resistant cultivars (Zhu, et al, 2009). For example, some salad vegetables could resist up to 4000 Gy without physical-chemical damages and considerable quality loss (Nunes et al, 2008) whereas, for some products, such as lettuce leaves and apple fruit (Gala and Fuji variety) radiation doses should not exceed 600 Gy (Dionísio et al, 2009;Niemira et al, 2002).…”